1. Field of the Invention
The present invention relates to a very high density memory apparatus which employs the principle of an STM (Scanning Tunneling Microscope) which is capable of measuring atomic level irregularities by detecting a tunnel current.
2. Related Background Art
There has been a great desire to realize a technology capable of performing high density recording in the information recording field. The desire of a large capacity memory is the central subject in the electronic industry such as in a computer, its relative equipment and the video disk. Accordingly, many studies have been done to meet the above-described desire.
Hitherto, magnetic recording has been ordinarily employed to record information of a large capacity. Recently, optical recording using laser beams and optomagnetic recording using both laser beams and a magnetic field have become available, causing the recording density to be further improved. Since the optical recording methods require a laser beam having a shorter wavelength in order to further raise the recording density, it is expected that it is very difficult to significantly improve the recording density.
On the other hand, a high resolution microscope called an "STM" (Scanning Tunnel Microscope) capable of directly observing atoms present on the surface of the conductor has been developed [G. Binning et al., Helvetica Physica Acta, 55, 726 (1982)]. As a result, a real spatial image can be observed at a high resolution regardless of whether or not the subject is monocrystal or amorphous. Furthermore, the STM exhibits an advantage in that the subject can be observed at reduced energy while eliminating a risk of damaging the medium by an electric current. In addition, since the STM is able to operate in atmosphere and to be applied to various materials, the STM is expected to be widely used.
The STM is arranged to utilize a fact that a tunnel current flows when a voltage is applied between a probe (a probe electrode) and a conductive material which are caused to come close to each other to a distance of about 1 nm. Since the tunnel current is very sensitive to the change in the distance between the probe electrode and the conductive material, the surface structure of the real space can be drawn by scanning the probe electrode in such a manner that the tunnel current is maintained at a constant value. Simultaneously, various information concerning all electron clouds of the surface atoms can be read out. At this time, the resolution in an in-plane direction is about 1 .ANG.. Therefore, by utilizing the principle of the STM, desired high density recording/reproducing can be performed in the atomic order (several .ANG.). As the recording/reproducing method, a method has been proposed in which the surface status of an appropriate recording layer is changed by using corpuscular beams (electron beams or ion beams), high energy electromagnetic waves such as X-rays or energy beams such as visible or ultraviolet rays to thereby perform recording. Thus, the STM is used to reproduce data. Another method has been proposed in which a material having an effect of memorizing the switching characteristics of an electric current, for example, a conjugate .pi. electronic organic compound or a material containing chalcogen is used to form a thin layer so as to perform recording/reproducing by the STM.
By using the above-described recording/reproducing methods, a memory exhibiting an extremely high density and a large capacity can be realized. However, when a great quantity of information is desired to be actually read out, the XY directional (in an in-plane direction of the recording medium) position detection of the probe and correction control (tracking) are required.
The tracking can be performed by a method in which the atomic arrangement of the recording medium is utilized to form the tracking signal to perform scanning with a probe electrode. Another method can be available in which a track is previously formed in the surface of the recording medium. Furthermore, a wobbling method can be employed in which the probe electrode is finely vibrated in the widthwise direction of the information bit line. In particular, the wobbling method is very simple and convenient in comparison to the other methods because the tracking signal can be generated from the reproduced signal of information.
The wobbling method will now be described.
In the wobbling method, when a recorded information bit line is scanned to read a reproduced signal, the probe electrode is stationarily vibrated at a frequency f with an amplitude smaller than the width of the bit line, in a direction perpendicular to the bit line. The frequency f is set to a value sufficiently large with respect to the frequency of the reproduced signal of the bit line. As a result, the amplitude of the reproduce signal of the bit line changes in accordance with the displacement between the probe electrode and the bit column as shown in FIG. 1A. That is, the amplitude intensity of the modulation signal becomes a maximum value when the probe electrode is positioned above the bit line as shown in the graph shown in FIG. 1A. On the contrary, the same is reduced when the probe electrode is moved away from the bit line. When the probe is vibrated finely at the frequency f, the envelope of the reproduced signal of the bit line is, as shown in FIG. 1B, changed as designated by signals b, c and d shown in FIG. 1B depending on the positions shown by the arrows given by same symbols in FIG. 1A. Therefore, by taking the signals denoting the changes in the envelope, signals b', c' and d' shown in FIG. 1B can be obtained. That is, the envelope change signal with respect to vibration waveform a of the probe electrode is reduced as designated by signal c', when the probe electrode is positioned above the bit line as designated by arrow c. When the same is displaced upwards as designated by arrow b, the amplitude is enlarged while the phase is displaced by 180.degree. with respect to the vibration waveform a of the probe electrode. When the same is displaced downwards as designated by arrow d, the amplitude is enlarged with the same phase as that of the vibration waveform a of the probe electrode. Therefore, by performing a phase detection operation using the normal signal of frequency f of the probe electrode as a reference signal, a tracking signal in proportion to the displacement quantity from the bit line can be obtained. As a result, a feedback control in which the probe electrode is maintained at a position above the bit column, can be performed by using the tracking signal.
According to the wobbling method, the wobbling frequency f must be higher than the offtrack frequency component in order to stably perform the tracking. On the contrary, a problem arises in that the S/N of the reproduced signal deteriorates in proportion to the vibration frequency of the probe electrode. That is, there is a contrary relationship between the stability of tracking and the S/N of the reproduced signal [for example, see "Collection of Integrated Technology of Optical Memory and Optomagnetic Memory"(Science Forum, 1983, p. 123) supervised by Yoshifumi Sakurai and Shizuo Tatsuoka].